DC-to-DC power converters are widely used to supply power to electronic devices, such as in computers, printers, etc. Such DC-to-DC converters are available in a variety of configurations for producing a desired output voltage from a source voltage. For example, a buck or step down converter produces an output voltage that is less than the source voltage.
A typical step down converter includes a high-side power switch which is pulse width modulated by a control circuit to thereby periodically connect a source voltage to an inductor and capacitor. The voltage developed across the capacitor powers the load. In addition, the output voltage is typically sensed, such as by a voltage divider, and fed as one input to an error amplifier. A reference voltage is fed to a second input of the error amplifier. The output of the error amplifier feeds one input of a comparator. The other comparator input is typically fed by a periodic control waveform, such as a triangle wave. The comparator, in turn, operates the power switch with a series of control pulses, the width of which are used to regulate the load voltage to the desired level despite fluctuations in the load. It is, of course, desirable that the power conversion efficiency be relatively high, and that the desired output voltage be accurately maintained.
Modern computer systems, for example, may require multiple DC power supplies. For example, the microprocessor core of a portable computer may require a separate and different supply voltage than other circuit portions. Accordingly, for such applications, several DC-to-DC converters having pulse modulation control as described above may be combined into a single circuit, and, more preferably, into a single integrated circuit. In addition, at certain times or for certain applications, two converters may need to deliver similar output voltages.
Unfortunately, if two PWM DC-to-DC converters are used in the same system, such as in the same integrated circuit or on the same circuit board, for example, it is possible for these converters to interact in a way that results in noisy or jittery operation of each of the converters. This may typically occur when the two converters are operating at nearly the same duty cycle. Under this condition, the power switch of one converter will turn on or off at a time that immediately precedes the turn-on or turn-off of the power switch of the other converter. When the first converter switches, noise is induced into the control circuitry of the second converter. This may result in the second converter switching prematurely. When the second converter switches prematurely, this may cause too much or too little output voltage to be generated, and the control circuitry will respond by further decreasing or increasing the duty cycle of the power switch. This sets up a jitter in the pulse width of the second converter.
Generally, the second converter will also interfere with the operation of the first converter, as well. The two converters may appear to lock to the same jittery duty cycle. The resulting operation causes undesirable noise, and, if the amplitude of the jitter is large enough, there may also be a large, low-frequency oscillation in the output voltages of the converters.
The problem with respect to mutual interference and jitter may be particularly pronounced when the error amplifier, comparator and triangle waveform generator for the two or more converters are integrated into a single integrated circuit. In particular, this may be a problem on a personal computer motherboard, which typically requires the generation of a fixed 3.3 V supply for the input/output (I/O) circuits, as well as a variable 1.3 to 3.5 V supply for the core of the microprocessor. If the adjustable core voltage is set to be relatively near the 3.3 V supply, then jitter may likely occur.
In the past, attempts have been made to address the jitter problem for multiple co-located PWM DC-to-DC converters by reducing the amount of noise coupled to the respective control circuits of the converters. This has been done by using separate grounds and power supplies for the two control circuits. Unfortunately, this approach is limited because there is always some noise coupled between the two control circuits, especially when they are on the same IC. This prior art approach also has the drawbacks of requiring extra power supply pins for the IC, and requiring extra decoupling capacitors. In addition, this prior approach is not always effective in removing jitter.